UserId is a built-in field in SSRS. If you look in the Expression editor window, you will see that listed under built in fields. At runtime, it will get populated with the userid that is running the report.

I am not familiar with the right(User!UserID,10) in the context of an RDL file, but I would guess that it is taking the rightmost 10 characters of the username. If you are using the Windows Authentication, the UserId is of the form Domain\Username. So perhaps, they are trying to just get the rightmost 10 characters to get the Username?